Photoelectric autocollimator is widely used for measurement and calibration of small angles, flatness measurement of plates, sway angle measurement of axises, straightness measurement of slides, and position uncertainty measurement of rotary tables, etc. in the fields of mechanical manufacturing, shipbuilding, aerospace, and Scientific Research, etc.
Laser is often used for long-distance and high-precision measurement of angles because of its monochromaticity and high energy density. Many high-precision photoelectric autocollimators have been developed with laser used as light source. But their measurement uncertainty is mainly limited by the drift of laser beam, which increases as the measurement distance increases.
The uncertainty of most photoelectric autocollimators is now above 0.5 arcsecond, and their measurement distance is usually less than 6 m. The drift of laser beam mainly comes from: (1) unstable light intensity and direction of light emitting; (2) atmospheric disturbance and random jittering during beam propagation; (3) light bending due to atmospheric refractive.
Restraining or compensating beam drift is the key to the improvement of measurement accuracy. The followings are some methods which can be used to restrain or compensate beam drift:
(1) Spatial connection lines of diffraction or interference fringes generated by zone plates, phase plates, binary optic elements or double slits, can be used to restrain beam drift. For example, a zone plate can be used to generate a bright cross reticle on the line connecting laser light source and zone plate center. The cross reticle can be imaged at different positions along the optical axis, by adjusting a telescope between laser light source and zone plate. So the optical axis can be used as a datum line and can be traced by the cross reticle. Because the reticle is generated by diffraction and interference, it has a good anti-interference performance and it can therefore reach a measurement accuracy of +/−1×10−7 rad (0.04 arcsecond). However, this method requires constant adjustment of the telescope to move the reticle along the optical-axis, and so, it cannot be used to achieve real-time compensation. For this reason, the application of this method is limited.
Richard F. et al. proposed a Poisson line method. A plane wave is used to illuminate an opaque sphere to generate a Poisson line perpendicular to the incident plane wave, and the line reverse extension cord passes through the sphere center. The line can thus be used as a datum line with the capability of rejection to interference. But the directional change of incident plane wave causes the directional change of Poisson line, which has a direct influence on the accuracy of measurement.
Hao Q. et al. proposed a collimation method which uses the central dark line generated by the diffraction of a phase plate as the datum line. If the incident light is a plane wave in a given direction, the position of dark line remains unchanged when the incident plane wave has a linear drift. In this way, the method restrains the beam drift and achieves a measurement accuracy of 10−6 rad (0.2 arcsecond). However, this method cannot be used to restrain the angular drift of incident light.
(2) Methods of dual beams compensation
Xingzhan Liu, et al. from Tsinghua University proposed a symmetrical dual beams compensation method. In this method, a special light path is used to split the incident beam into two symmetrical beams. The linear or angular drift of incident beam causes directional changes of the two symmetrical beams, while the direction of symmetrical center of the two beams remains unchanged. This method can be used to achieve a measurement accuracy of 1.8×10−6 rad (0.37 arcsecond). However, the generation of symmetrical dual beams needs multiple reflections and refractions, which increase the difficulties of processing and installing optic components. In addition, the two symmetrical beams have different drifts due to the noncoincidence of two beam paths, which restrains the effect of compensation for the drift caused on the beam path.
Cuifang Kuang from Beijing Jiaotong University proposed a common path compensation system. The emergent laser beam is parallelly reflected back by a cube-corner and divided into two beams by a prism. One of the beams is used as the measurement signal and the other is used as the compensating signal to compensate the position error resulting from air disturbance. The angular drift caused by air disturbance can be compensated using this method in real time. But this method is mainly appropriate for measurement of straightness, and it is difficult to use it for measurement of angles, because of the parallel and reversely reflective properties of a cube-corner reflector.
Fengling You from Beijing Jiaotong University proposed an angle measurement method based on common-path compensation for beam drift. In this method a semi-reflective mirror and a cube-corner reflector are used as moving parts which move along the slide to be measured. The semi-reflective mirror divides the incident beam into a reflected beam and a transmitted beam. The reflected beam is used as the measurement beam, and the transmitted beam is reflected back by the cube-corner reflector and used as the compensation beam to gain the angular drift in the measuring process for real-time compensation to improve the measurement accuracy. In this method the measurement beam and the compensation beam are not on a common path during returning. The drift of compensation beam can not completely represent the drift of measurement beam, which leads to a poor compensation effect.
(3) Method of close-loop feedback control
Close-loop feedback control can be used to enhance the directional stability of laser beam. This method provides an effective technical approach to eliminate or compensate the angle measurement error resulting from beam drift to achieve a high angle measurement accuracy.
Dianhong Yu, et al. from Xi'an University of Technology adopted the method of close-loop feedback control to restrain beam drift. The feedback system receives the beam drift signal and drives the actuating mechanism to adjust the two-dimensional direction of laser beam. This method can be used to achieve a real-time correction for beam drift and an accuracy of 5×10−7 rad (0.1 arcsecond). But this method is used to enhance the directional stability of laser beam only, and it cannot be used for measurement of angles.
Jiubin Tan, et al. proposed in 2004 a laser beam alignment system based on fast feedback control. The system can be used to dynamically detect and control the linear and angular drifts of laser beam in real-time and achieve an alignment accuracy of 0.6×10−7 rad (0.01 arcsecond) in a given direction. However, this system can only used for alignment of laser beam in a given direction. It can not be used for measurement of angles.
Jiubin Tan, et al. proposed in 2005 a photoelectric autocollimation method and apparatus (Patent application key: ZL200510089852.3). A special beam splitting target is used in this method to feedback a reference beam with the same characteristics of drift as the measurement beam during measurement of angles. The drift of measurement beam is restrained by controlling the beam steering device using the beam drift signal taken from the reference beam. This method can be used to enhance the stability of a photoelectric autocollimator over a large distance. However, the measurement and reference beams used in this method are not on a common path during returning. The two beams return from different paths, and so, the drift of reference beam cannot completely represent the drift of measurement beam. The difference of the drifts of the two beams increases while the measurement distance increases. The feedback control system cannot effectively restrain the drift of measurement beam, and so, the final angle measurement accuracy is restrained.
In conclusion, the existing methods and apparatuses described above have the following deficiencies:
(1) The method of using spatial connection lines of diffraction or interference fringes requires high directional stability of the incident beam, and it cannot restrain the angular drift of the incident beam. The angular drift of incident beam results to the drift of the spatial connection lines which is used as the datum line. Moreover, this method has a limited capability of restraining the air disturbance in the beam path.
(2) The generation of the symmetrical dual beams in the methods of dual beams compensation is difficult. And the measurement and reference beams used in this method are not on a common path, and so, the drift of reference beam cannot completely represent the drift of measurement beam, which results to poor compensation effect.
(3) The close-loop feedback control method is mainly used to enhance the stability of laser beam in a given direction, and it is difficult to be used for measurement of angles. Moreover, the reference and measurement beams cannot transmit on a common path, and so the drift of reference beam is not completely the same as the drift of measurement beam, which results to poor compensation effect.